(i) Field of the Invention
The present invention relates to an apparatus for computing the relative velocity between sprung and unsprung structures in a vehicle connected through a shock absorber and a spring.
(ii) Description of the Related Art
A suspension of a vehicle is a system for connecting a sprung structure, such as a chassis, with an unsprung structure such as a wheel, which supports the sprung structure and greatly influences vibration, ride comfort, and maneuvering stability. The suspension includes such parts as a spring for moderating a shock from the surface of a road and a shock absorber for damping the free vibration of the spring and controlling the velocity of an attitude vary of a vehicle. As described above, the suspension has various requirements. However, these performances frequently conflict with each other. Therefore, in the case of design, it is desirable to find a certain compromising point by considering the balance between these performances.
In order to achieve the suspension's requirements to a high degree, an art is known which makes it possible to vary the characteristics of a suspension. For example, there is an art for varying damping forces of a shock absorber according to the state of a vehicle. In this example, a damping force is increased so as to early converge the free vibration of a spring system in the region of the resonance frequency of a sprung structure and the region of the resonance frequency of an unsprung structure and the damping force is decreased in regions other than the above regions so as to greatly absorb by the suspension the input from the road surface.
To control the above variable damping-force suspension and to analyze movement of the suspension, an apparatus for estimating the position and velocity of the suspension and parameters serving as other physical values has been proposed. For example, in Japanese Patent Laid-Open Publication No. Hei 6-106937, a suspension having a variable damping-force shock absorber is controlled by estimating a certain parameter showing the vertical velocity of a sprung structure and assuming that the damping force of the shock absorber is generated proportionally to the parameter. Moreover, according to the art disclosed in the official gazette, a single-wheel model with two degrees of freedom is used to estimate the parameter. That is to say, the model is used in which a sprung structure and an unsprung structure having a limited mass are connected each other by a spring and a shock absorber to input an external force from the unsprung structure through the spring element of a wheel.
However, when estimating a vibration having a frequency component close to the resonance frequency of a sprung structure, it is possible to ignore the motion of an unsprung structure because the amplitude of the unsprung structure is significantly small compared to that of the sprung structure. That is to say, it is possible to use a model with one degree of freedom instead of the above model with two degrees of freedom. In other words, the above-described conventional estimating apparatus uses an excessively large model in order to analyze a point nearby the resonance point of a sprung structure and it therefore has a problem that the operation load increases.